With a known method of pouring molten metal from a ladle into a tundish in continuous casting of metal, a single-opening nozzle is used and the liquid level of the tundish is controlled by a so-called throttling pouring which changes the opening area of the nozzle. A disadvantage of this prior art method is that the molten metal poured by the throttling pouring results in an unbrellalike stream as will be described later thus inevitably causing oxidation of the poured molten metal stream by air, and moreover the most serious drawback of the throttling pouring will be a reduced life of the refractory brick, i.e., the fixed plate brick, sliding plate brick, etc., due to the damages to their portions around the nozzle opening.
The inventor, etc, have discovered by experiments that the prior art throttling pouring with a single-opening nozzle causes formation of a large amount of large-sized inclusions in cast slabs and that the wide open pouring without any throttling operation has the effect of very greatly reducing the trapping of air in the poured molten metal stream and thereby preventing oxidation of the pouring stream by the trapped air and reducing the formation of large-sized inclusions in cast slabs. This is believed to result from the following reasons. In other words, with the prior art pouring method, a throttling pouring is employed in which the nozzle openings of a fixed plate and sliding plate are selected practically the same or different from each other and the centers of the nozzle bores are offset to vary the effective opening area. As a result, when the poured molten metal stream passes through the offset portions, the molten metal stream is caused to eddy in the vicinity of the offset portions so that melting loss of the refractory bricks will be promoted and the flow line of the poured molten metal stream will be disturbed, thus affecting the molten metal stream just below the nozzle and thereby disturbing the molten metal stream. On the contrary, the wide open pouring is believed to cause no trapping of air in the molten metal stream, since the wide open pouring does not cause any eddy of the molten metal and disturbance of the flow line and the molten metal stream flows as an undisturbed stream. These conditions will be explained with reference to FIG. 11 which is a traced copy of a photograph taken by a high speed camera and showing the flowing conditions of the poured molten metal stream just below the ladle nozzle, with FIG. 11(A) showing the conditions obtained with a wide open nozzle having a nozzle diameter of 50 mm.phi. and FIG. 11(B) showing the conditions obtained with a throttled (70%--open), nozzle having a nozzle diameter of 70 mm.phi.. As will be seen from these Figures, there was no disturbance of the molten metal stream in the case of the wide open pouring, and the throttled pouring caused a considerable disturbance of the molten metal steram.
In summary, it is essential for the production of good quality cast slabs to use a rotary nozzle assembly having a plurality of nozzles of different diameters which meet the requirements for ensuring the required pour throughout the pouring period when pouring molten metal from a ladle into a tundish and to effect the pouring by selectively opening fully these nozzles and causing the molten metal to flow as a normal flow, and the previously mentioned essence of this invention attributes to this point.